The present invention relates to tire testing machines utilizing a flat belt to simulate a road surface, and in particular, the invention relates to apparatus and methods for accurately calibrating such machines.
Generating an improved tire design requires reliable testing equipment to measure tire performance under various load conditions. Tire testing machines have been used to evaluate tires under various load conditions to accurately determine the performance of various tire designs, under numerous conditions of tread wear, speed, load, and the like. Such testing may include low force static testing or high force dynamic testing.
One testing machine that has proved useful in such development work is the MTS Flat-Trac(copyright) machine which is sold commercially by MTS Systems Corporation of Minneapolis, Minn. Such machines are adapted for testing passenger car and light truck tires under a wide range of conditions. This particular testing machine, and other machines like it, use a simulated flat surface roadway. The machines control speed, radial force, radial position, slip angle, camber angle, inflation pressure, and spindle torque. Such testing machines facilitate the measurement of various forces acting upon the tire.
To obtain accurate data on tire performance, it is absolutely critical that the force sensing mechanism within the machine, comprising an electronic transducer, accurately measure and record the forces and moments exerted during testing in three axesxe2x80x94the X axis, Y axis and Z axis. Accurate measurement of force and moment vectors in each of the three axes depends upon a reliable and precise means and apparatus for performing a force and moment transducer calibration to generate an internal matrix data set that can be used by the machine controller in calculating corrected results. This corrective data set, or correction matrix, then may be used to compensate for crosstalk between the various transducer elements (or parts) within the force and moment transducer during testing.
At least two United States patents are directed to flat belt tire tester machines that provide for mounting a wheel and tire to be tested about an axis positioned above a flat rotating belt. U.S. Pat. Nos. 4,238,954 and 4,344,324 to Langer (collectively the xe2x80x9cLanger patent(s)xe2x80x9d) each are directed to such a flat belt tire tester.
Multi-axis transducers have the capability to provide five or six measurements including moments and axial force measurements in the three principal axes (X, Y and Z). Such transducers are known in the art. However, there is a need in such applications to reduce crosstalk, i.e. the undesirable changes in reading for one axis resulting from loads applied in another axis.
In the past, calibration typically has been accomplished by applying five load vectors (six vectors when the spindle drive option is installed) to the transducer while measuring and recording the crosstalk produced by the loads applied. Then, a computational method has been employed to generate a calibration matrix file and interaction matrix from the recorded parameters.
Unfortunately, existing means of calibration have been significantly limited in the degree of precision that can be obtained in the measurement of forces acting upon the transducer, thus limiting the overall accuracy and reliability of the machine and its measurement of tire forces and tire responses. It is fundamental that a machine measurement can be only as accurate as the means used to calibrate the machine.
There are several disadvantages with such calibration methods. In the Mz axis calibration, there is an undesirable longitudinal force applied simultaneously with the aligning torque during calibration, so that the measurement taken during calibration is not a pure measurement. In the Fz axis calibration, there is a mechanical coupling that may undesirably permit Mz torque to be introduced, through friction, creating an erroneous value for Mz crosstalk compensation.
What is needed in the industry is a calibration method and apparatus that provides the highest possible level of accuracy and the slightest possible chance of introducing erroneous off-axis (i.e. crosstalk) forces or moments during the calibration procedure.
The present invention relates to improving tire testing machine accuracy through improved calibration apparatus and methods. Improvements to existing apparatus for calibrating a testing machine transducer are provided in the invention.
The transducer is capable of measuring load forces and moments in three perpendicular intersecting axes, the first axis comprising a longitudinal axis, the second axis comprising a lateral axis, and the third axis comprising a steer axis. Furthermore, a spindle with extension is aligned along the lateral axis and operably connected to the transducer.
During calibration, a spindle extension is connected to the spindle, which is located within the housing of the tire testing machine. The spindle extension includes a first location corresponding to the spindle axis/steer axis intersection point and a second location upon the spindle axis. The second location is typically provided at a predetermined distance from the first location. In the invention, an improved procedure for calibrating aligning torque or moment around the z axis (i.e., steer axis) (Mz) is provided. This method employs a mechanical couple.
In one application of the invention, first and second load members are provided, each of the first and second load members having a proximal end adapted for connection to the spindle and a distal end adapted for receiving a predetermined weight. The first load member is connected to the spindle extension at the first location, and the second load member is connected to the spindle extension at the second location. The first load member and the second load member together comprise a mechanical couple, such that a first force exerted upon the first load member by a first weight, and a second force exerted upon the second load member by a second weight, are intended to be equal. Thus, the first force and the second force act equally, but in opposite directions.
Furthermore, in other aspects of the invention, an improved method for isolating the normal force along the steer axis (i.e., Fz) is provided. In the latter case, an air bearing is used in connection with a load cell strut, wherein the load cell strut has been adapted for transmitting forces along the steer axis between the spindle and the air bearing. A friction reduction means is adapted for reducing the frictional contact between the load cell strut and the spindle. An air bearing comprising a load cell, a load cell strut, and a friction reduction means (or low-friction yoke) is adapted for reducing frictional contact between the load cell strut and a spindle extension.